Interpretive Summary: Over time, many single, all-stage resistance genes to stripe rust in wheat are circumvented by race changes in the pathogen. In contrast, high-temperature, adult-plant resistance (HTAP), which only is expressed during the adult-plant stage and when air temperatures are warm, provides durable protection against stripe rust. Our objective was to identify major quantitative trait loci (QTL) for HTAP resistance to stripe rust in the spring wheat cultivar ‘Louise’. The mapping population from a Louise (resistant) by ‘Penawawa’ (susceptible) cross was used in the experiemnets for disease evaluation and marker identification. One major QTL, designated QYrlo.wpg-2BS, associated with HTAP resistance in Louise, was detected on chromosome 2BS within a 16.9 cM chromosomal region flanked by molecular markers Xwmc474 and Xgwm148. SSR markers associated with QYrlo.wpg-2BS are currently being used in marker-based forward breeding strategies to transfer the target region into adapted germplasm to improve the durability of resistance in resulting cultivars.

Technical Abstract:
Over time, many single, all-stage resistance genes to stripe rust (Puccinia striiformis f. sp. tritici) in wheat (Triticum aestivum L.) are circumvented by race changes in the pathogen. In contrast, high-temperature, adult-plant resistance (HTAP), which only is expressed during the adult-plant stage and when air temperatures are warm, provides durable protection against stripe rust. Our objective was to identify major quantitative trait loci (QTL) for HTAP resistance to stripe rust in the spring wheat cultivar ‘Louise’. The mapping population consisted of 188 recombinant inbred lines (RILs) from a Louise (resistant) by ‘Penawawa’ (susceptible) cross. F5:6 lines were evaluated for stripe rust reaction under natural infection in replicated field trials at five locations in the U.S. Pacific Northwest in 2007 and 2008. Infection type (IT) and disease severity (DS) were recorded for each RIL two to four times per location. In all environments, Penawawa was rated with an IT ranging from 6 to 8 at all growth stages evaluated. In contrast, Louise, the resistant parent, was rated with an IT of 2 or 3 across growth stages. Distribution of IT values was bimodal, indicating a single major gene was affecting the trait. The parents and RIL population were evaluated with 295 polymorphic simple sequence repeat (SSR) and one single nucleotide polymorphism (SNP) markers. One major QTL, designated QYrlo.wpg-2BS, associated with HTAP resistance in Louise, was detected on chromosome 2BS (LOD scores ranging from 5.5 to 62.3 across locations and years) within a 16.9 cM region flanked by Xwmc474 and Xgwm148. SSR markers associated with QYrlo.wpg-2BS are currently being used in marker-based forward breeding strategies to transfer the target region into adapted germplasm to improve the durability of resistance in resulting cultivars.